A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits.
It has been proposed to immerse the substrate in the lithographic projection apparatus in an immersion fluid, e.g. a liquid having a relatively high refractive index, e.g. water, so as to fill a space between a final element of a projection system and the substrate. The immersion fluid may be distilled water, although another fluid can be used. An embodiment of the invention will be described with reference to immersion fluid. Many fluids may be suitable, particularly a wetting fluid, an incompressible fluid and/or a fluid with higher refractive index than air, desirably a higher refractive index than water. Fluids excluding gases are particularly desirable. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the fluid. (The effect of the fluid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion fluids have been proposed, including water with solid particles (e.g. quartz) suspended therein, or a liquid with a nano-particle suspension (e.g. particles with a maximum dimension of up to 10 nm). The suspended particles may or may not have a similar or the same refractive index as the liquid in which they are suspended. Other liquids which may be suitable include a hydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueous solution.
In an immersion apparatus, immersion fluid is handled by an immersion system, device, structure or apparatus. In an embodiment the immersion system may supply immersion fluid and may be referred to as a fluid supply system. In an embodiment the immersion system may at least partly confine immersion fluid and may be referred to as a fluid confinement system. In an embodiment the immersion system may provide a bather to immersion fluid and thereby be referred to as a barrier member, such as a fluid confinement structure. In an embodiment the immersion system creates or uses a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the immersion system may comprise a fluid handling structure, which may be referred to as a seal member, to provide the flow of gas. In an embodiment, immersion liquid is used as the immersion fluid. In that case the immersion system may be a liquid handling system.
However, use of an immersion system may lead to defects forming on a top surface of the substrate. Defects can be caused by a droplet of immersion fluid being left behind after the substrate passes under the fluid handling structure. In particular, at least two main mechanisms which result in defects are known, which are called bulldozing and film pulling. Defects on the surface of the substrate may lead to errors on the surface of the substrate which can reduce yield. Defects may mean watermarks in particular, or may mean other defects which may occur on the surface of the substrate.
Film pulling may occur as the substrate is moved relative to an immersion system (such as the fluid handling structure or the like). As the surface of the substrate moves relative to the immersion fluid, any variation (such as an edge of the substrate) or irregularity on the surface of the substrate may act as a meniscus pinning feature as the immersion fluid passes over it. This means that as the fluid handling structure moves relative to the substrate, the meniscus of the immersion fluid between the surface of the substrate and the fluid handling structure is stretched. After the fluid handling structure has moved a certain distance, the meniscus will eventually break and immersion fluid is left on the surface of the substrate resulting in droplets on the substrate which can lead to watermark defects. The remaining droplets may thus lead to errors on the surface of the photosensitive material which can reduce yield. Film pulling may be reduced by increasing the gas flow of a gas knife at a receding side of the fluid handling structure. However, this may have other consequences at an advancing side of the fluid handling structure. For example, using an increased gas flow for the gas knife will increase “bulldozing” at the advancing side of the fluid handling structure as described below.
Bulldozing may also occur when the substrate is moved relative to the fluid handling structure. Bulldozing occurs when a droplet of immersion fluid is encountered which is ahead of the fluid handling structure. As the substrate moves, the advancing portion of the fluid handling structure collides with the droplet of immersion fluid and the droplet is pushed forwards by the fluid handling structure. As the droplet is pushed forward, defects are created on the surface of the substrate. Although this may be effectively reduced by reducing the gas flow of a gas knife at the advancing side of the fluid handling structure, this may have other consequences. For example, using a reduced gas flow for the gas knife may mean that the confined immersion fluid is more likely to escape from the fluid handling structure at the receding side of the fluid handling structure, thus leading to further defects.